1. Field of the Invention
The present invention relates to a method for operating a semiconductor device. More particularly, the present invention relates to a method for programming a P-channel electrical erasable programmable read-only memory (EEPROM).
2. Description of the Related Art
Electrical erasable programmable read-only memory (EEPROM) is a type of non-volatile memory with many advantages including a rapid response, a large storage capacity and a small size. Hence, EEPROM has become one of the most frequently used portable memory devices. Earlier EEPROM was constructed of N-type transistors and programmed with channel hot electron injection (CHEI). However, the CHEI programming method has low electron injection efficiency. Hence, newer EEPROM is constructed of P-channel transistors and programmed with band-to-band tunneling hot electron injection (BTBTHEI). The BTBTHEI method has an electron injection efficiency roughly two magnitudes higher than that of the CHEI method, and hence makes a much higher programming speed.
FIG. 1 is a schematic cross-sectional view of a P-channel EEPROM undergoing a programming operation using a conventional BTBTHEI method. The P-channel EEPROM comprises an N-well 100, a floating gate 110, a control gate 120, a select gate 130, a P-type source region 140, a P-type drain region 150 and a P-doped region 160 coupled to a bit line (not shown). To program the P-channel EEPROM, a negative voltage Vd is applied to the bit line/P-doped region 160, and another negative voltage Vsg is applied to the select gate 130 to turn on the channel below so that the negative voltage is passed to the drain region 150. In the meantime, a high positive voltage Vcg is applied to the control gate 120 to induce band-to-band tunneling hot electrons under the floating gate 110 close to the drain region 150, and a part of the hot electrons are attracted into the floating gate 110. Furthermore, the source region 140 is floated in the conventional BTBTHEI method.
However, both the CHEI programming method for N-channel EEPROM and the BTBTHEI programming method for P-channel EEPROM need relatively high operation voltages. FIG. 3 is a graph showing the variation of threshold voltage of a memory cell with time at different bit line voltage Vd using the conventional BTBTHEI method for programming a P-channel EEPROM. As shown in FIG. 3, the control gate voltage Vcg is set to a constant voltage of 7V. To generate sufficient band-to-band tunneling hot electrons so that programming can be completed within a designated period, a negative voltage Vd up to −7V, rather than −5V or −3V, must be applied to the bit line. With the application of such a high negative voltage, reducing the channel length of the select gate 130 is unfeasible due to the possibility of an increase in punch-through leakage.
Furthermore, to increase the amount of hot electrons and the percentage of injected hot electrons for speeding up the programming operation, usually a negative voltage Vsg up to −8V to −9V is applied to the select gate 130, and a positive voltage Vcg up to 8V to 9V is applied to the control gate 120. Hence, the overall power consumption during a memory programming operation is relatively high in the prior art.